1. Field of the Invention
This invention relates to a semiconductor device made of a plurality of components, including a voltage generating circuit.
2. Description of the Background Art
A semiconductor integrated circuit, such as a microcomputer or the like, includes a voltage down converter (VDC) for lowering an external power supply voltage to generate an internal power supply voltage, in order to supply a drive voltage to an internal circuit. The voltage down converter is usually constituted of a reference voltage generating circuit for generating a reference voltage and an internal power supply voltage generating circuit having a negative feedback loop. A representative reference voltage generating circuit is a band gap reference (BGR) circuit for generating a reference voltage of approximately 1.1 V by utilizing the band gap of silicon.
The internal power supply voltage is set to, for example, 1.5 V, in accordance with the product specifications. Although the reference voltage generating circuit and the internal power supply voltage generating circuit are designed so as to achieve this voltage set value, the internal power supply voltage sometimes deviates from the value at the time of design (design value), owing to variations in the manufacturing process. In particular, as a semiconductor integrated circuit becomes smaller, variations in characteristics of a circuit component becomes greater, thus inevitably resulting in an error between an actual value and the design value. For this reason, each of the reference voltage generating circuit and the internal power supply voltage generating circuit is provided with a trimming function for correcting such an error.
According to a typical trimming technique, the internal power supply voltage is measured by an analog-to-digital converter (ADC) incorporated in a semiconductor integrated circuit device, and trimming of the reference voltage generating circuit is performed such that the internal power supply voltage is set to a desired value (for example, 1.5 V).
A reference voltage outputted from the reference voltage generating circuit may be detected by the ADC in place of the internal power supply voltage. In this case, however, a voltage follower circuit needs to be provided to perform impedance conversion of an output from the reference voltage generating circuit. This is because, due to an insufficient charge supplying capability of the reference voltage generating circuit, directly connecting the reference voltage generating circuit and the ADC without a voltage follower circuit being interposed therebetween causes the reference voltage to change due to charge sharing.
Additionally, charge sharing often becomes a problem in a dynamic logic circuit, and therefore, methods for avoiding charge sharing have been proposed. For example, in the method described in Japanese Patent Laying-Open No. 2004-289641, in order to prevent supplied charges from being transferred to a neighboring node at the time of pre-charging a line in a logic circuit, a node to which charges may be transferred from the pre-charged line is also pre-charged. Japanese Patent Laying-Open No. 2001-274677 discloses a cross-coupled load-type dynamic logic circuit. In this circuit, charges that have been stored at a dynamic node may be transferred to a sub-dynamic node to which a plurality of load transistors are connected. Thus, the sub-dynamic node is also pre-charged at the time of pre-charging the dynamic node.
Conventional techniques for trimming a reference voltage generating circuit have problems with accuracy in measurement of the reference voltage. That is, when trimming of the reference voltage generating circuit is performed based on a result of observing the internal power supply voltage, the observed data of the internal power supply voltage includes not only an error in the reference voltage generating circuit, but also that in the internal power supply voltage generating circuit. In the case of the method wherein the reference voltage is measured by way of a voltage follower circuit, observed data includes the influence of offset of the voltage follower circuit. In either case, since the accuracy in measurement of the reference voltage is limited, it is difficult to adjust the reference voltage to a desired value.
As already described, if the reference voltage generating circuit and ADC are directly connected to each other without a voltage follower circuit being interposed therebetween, the influence of offset of the voltage follower circuit can be avoided; however, the reference voltage changes due to charge sharing. Changes in the reference voltage also causes great changes in the internal power supply voltage generated based on the reference voltage, thus possibly causing the internal circuit that operates on the internal power supply voltage to run away out of control or be broken down.